1. Field of the Invention
This invention relates to the control of permeability in subterranean oil-bearing formations.
2. Discussion of Related Art
In the production of oil from subterranean oil reservoirs by various flooding techniques, especially waterflooding, it has become a common expedient to add various polymeric thickening agents to the water in order to decrease its mobility to a point where it approaches that of the crude oil which is to be displaced so as to improve the displacement of the oil from the reservoir. Use of polymers for this purpose is often stated to be for mobility control.
Another problem which arises in the various flooding processes is that different strata or zones in the reservoir often possess different permeabilities so that displacing fluids enter the high permeability or "thief" zones in preference to zones of lower permeability where significant quantities of oil may be left unless measures are taken to at least partially plug the high permeability zones to divert the displacing fluid into the low permeability zones. Mechanical isolation of the thief zones has been tried but vertical communication among reservoir strata often renders such measures ineffective. Physical plugging of the high permeability zones by cements and solid slurries has also been attempted with varying degrees of success but the most serious drawback of this approach is the possibility of permanently closing productive portions of the reservoir.
From these early experiences, the desirability of designing a viscous slug capable of sealing off the most permeable layers so that the floodwater or other driving fluid, such as gas or steam, would be diverted to the underswept, tighter regions of the reservoir, became evident. This led to the use of oil/water emulsions, gels and polymers for controlling the permeability of the formations in a process frequently called "profile control" or "flood conformance", a reference to control of the vertical permeability profile of the reservoir. Profile control agents which have been proposed have included oil/water emulsions, gels, e.g., lignosulfonate gels and polymers, with polymers being the most extensively applied in recent years.
Among the polymers so far examined for improving flood conformance are polyacrylamides, polysaccharides, celluloses, furfural-alcohol and acrylic/epoxy resins, silicates and polyisocyanurates. A major part of this work has been conducted with the polyacrylamides, both in their normal, noncrosslinked form as well as in the form of metal complexes, as described, for example, in U.S. Pat. Nos. 4,009,755; 4,069,869 and 4,413,680. In either form, the beneficial effects derived from these polyacrylamides seem to dissipate rapidly due to shear degradation during injection and sensitivity to reservoir brines, low pH and high temperature. To overcome these problems and to achieve deeper penetration into the reservoir, dilute solutions of these polymers have sometimes been injected first and then complexed in situ.
Another group of polymeric thickeners which has received considerable attention for use in improving flooding are polysaccharides, particularly those produced by the action of bacteria of the genus Xanthomonas on carbohydrates. For example, U.S. Pat. Nos. 3,757,863 and 3,383,307 disclose a process for mobility control by the use of polysaccharides. U.S. Pat. Nos. 3,741,307; 4,009,755; 4,069,869 disclose the use of polysaccharides in the control of reservoir permeability. U.S. Pat. No. 4,413,680 describes the use of cross-linked polysaccharides for selective permeability control in oil reservoirs.
U.S. Pat. No. 3,908,760 describes a polymer waterflooding process in which a gelled, water-soluble Xanthomonas polysaccharide is injected into a stratified reservoir to form a slug, band or front of gel extending vertically across both high permeability and low permeability strata. This patent also suggests the use of complexed polysaccharides to block natural or man-made fractures in formations. The use of polyvalent metal ions for cross-linking polysaccharides is also disclosed in U.S. Pat. No. 3,810,882.
The use of various block copolymers for mobility control in waterflooding operations is described in U.S. Pat. Nos. 4,110,232, 4,120,801 and 4,222,881, but their use for permeability control has not been suggested.
Chung et al, in the aforementioned patent application Ser. No. 696,952, disclose the use of block copolymers, which may be cross-linked with polyvalent metal ions, as permeability control agents in enhanced oil recovery applications. However, polyvalent metal ions may not be effective cross-linking agents under all conditions encountered in the enhanced oil recovery applications, e.g., in acidic conditions, such as those encountered in carbon dioxide (CO.sub.2) flooding operations.
There is therefore a continuing need for different types of polymer gels which are effective as permeability control agents in different types of reservoirs under diverse reservoir conditions, including acidic conditions.